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94 Chapter 17: Rendering
Scene rendered without global illumination
Same scene with global illumination
Global illumination made smoother by final gather
Tocalculateglobalillumination,thementalray
renderer uses the photon map technique (page
3–994).
Impor tant: The mental ray renderer generates global
illumination without requir in g you to gen erate a
radiosity solution. A photon map is a model of global
illumination in its own right.
Note:
In order to use global illumination in mental
ray, the photons must be able to bounce among
two or more surfaces. This can be accomplished
by having a single object with some concavity in
its surface that’s exposed to the light source, or at
leasttwoobjects,andatleastoneobjectmustbe
set to receive global illumination (see mental ray
Panel (Object Properties D ialog) (page 1–126)).
Otherwise you’ll receive er ror messages and no
photons will be stored.
Using a photon map can cause rendering ar tifacts
such as dark corners and low-frequency variations
in the lighting. You can reduce or eliminate
these artifacts by turning on final gathering (page
3–940), which increases the number of rays used
to calculate global illumination.
You enable global illumination and final gathering
on the Render Scene dialog > Indirect Illumination
panel > Caustics And Global Illumination rollout
(page 3–106) . In addition, you must designate:
• Which light objects can generate global
illuminat ion.
• Which renderable objects can generate global
illuminat ion.
• Which renderable objects can receive global
illuminat ion.
The settings for generating and receiving global
illumination are on the Object Properties dialog >
mental ray Panel (page 1–126).